All references cited in this specification, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features, and/or technical background. More specifically, U.S. Pat. No. 7,741,921 “Trigger-Mode Distributed Wave Oscillator” (TMDWO), US Pub. 2012/0169427, “Force-Mode Distributed Wave Oscillator and Amplifier Systems” (FMDWO), and U.S. application Ser. No. 12/374,165 “Pumped Distributed Wave Oscillator System” (PDWO) are incorporated by reference which form the core oscillators of this application.
Disclosed is a Traveling Wave Based THz Signal Generation System [TWSGS] and Method of use thereof. It is well-known that THz signals are electromagnetic waves at frequencies in the trillion (1×1012) cycles per second or terahertz range from 0.3 to 3 THz. The term applies to electromagnetic radiation with frequencies between the high-frequency edge of the millimeter wave band, 300 gigahertz (3×1011 Hz), and the low frequency edge of the far-infrared light band, 3000 GHz (3×1012 Hz). Corresponding wavelengths of radiation in this band range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the sub-millimeter band, and its radiation as sub-millimeter waves.
Since Terahertz radiation falls in between infrared radiation and microwave radiation in the electromagnetic spectrum, it shares some properties with each of these. Like microwave radiation, terahertz radiation can penetrate a wide variety of non-conducting materials. Terahertz radiation can pass through clothing, paper, cardboard, wood, masonry, plastic and ceramics. The penetration depth is typically less than that of microwave radiation. Terahertz radiation has limited penetration through fog and clouds and cannot penetrate liquid water or metal. Like infrared and microwave radiation Terahertz radiation travels in a line of sight and is non-ionizing. Being non-ionizing, it is generally not harmful to human beings. Terahertz radiation can penetrate fabrics and plastics, so it can be used in surveillance, such as security screening, to uncover concealed weapons on a person, remotely. Other areas of promising research are medical imaging, spectroscopy in chemistry and biochemistry; potential uses in high-altitude telecommunications between aircraft and satellites avoiding the problem of terahertz signals being absorbed in the atmosphere. Other possible uses of terahertz sensing and imaging are proposed in manufacturing, quality control, and process monitoring. These in general exploit the traits of plastics and cardboard being transparent to terahertz radiation, making it possible to inspect packaged goods.
Though the potential is there for the uses of Terahertz radiation, technology for generating and manipulating it is in its infancy, and is a subject of active research. It represents the region in the electromagnetic spectrum that the frequency of electromagnetic radiation becomes too high to be measured by directly counting cycles using electronic counters, and must be measured by the proxy properties of wavelength and energy. Similarly, in this frequency range the generation and modulation of coherent electromagnetic signals ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, and requires new devices and techniques. In mid-2007, scientists at the U.S. Department of Energy's Argonne National Laboratory, along with collaborators in Turkey and Japan, announced the creation of a compact device that can lead to portable, battery-operated sources of T-rays, or terahertz radiation. (See Science News: “New T-ray Source Could Improve Airport Security, Cancer Detection,” 27 Nov. 2007). As described in Science News, this new T-ray source uses high-temperature superconducting crystals grown at the University of Tsukuba, Japan. These crystals comprise stacks of Josephson junctions that exhibit a unique electrical property: When an external voltage is applied, an alternating current will flow back and forth across the junctions at a frequency proportional to the strength of the voltage; this phenomenon is known as the Josephson effect. These alternating currents then produce electromagnetic fields whose frequency is tuned by the applied voltage. Even a small voltage—around two millivolts per junction—can induce frequencies in the terahertz range according to the article. In 2008, engineers at Harvard University demonstrated that room temperature emission of several hundred nanowatts of coherent terahertz radiation could be achieved with a semiconductor source. THz radiation was generated by nonlinear mixing of two modes in a mid-infrared quantum cascade laser. Until then, sources had required cryogenic cooling, greatly limiting their use in everyday applications. In 2009, it was shown that T-waves are produced when unpeeling adhesive tape. The observed spectrum of this terahertz radiation exhibits a peak at 2 THz and a broader peak at 18 THz. The radiation is not polarized. The mechanism of terahertz radiation is tribocharging of the adhesive tape and subsequent discharge. (“Peeling Adhesive Tape Emits Electromagnetic Radiation At Terahertz Frequencies,” www.opticsinfobase.org.). In 2011, Japanese electronic parts maker Rohm and a research team at Osaka University produced a chip capable of transmitting 1.5 Gbit/s using terahertz radiation. (“New Chip Enables Record-Breaking Wireless Data Transmission Speed,” www.techcrunch.com,)
As mentioned earlier, though the potential is there for very attractive uses of Terahertz radiation, technology for generating and manipulating it is still in its infancy. There have been several approaches to achieve radiative signals in the THz levels. In its formative years, this was accomplished by using various techniques including thin-film superconductive circuitry incorporating Josephson tunnel junctions; diode based multipliers; varactor diode type multiplier by cascading diode multipliers; harmonic-mixing technique to generate high frequency oscillation signals; sub-harmonic mixing; mixing frequency multipliers; sub-harmonic mixing with Doppler effects; combining equally spaced multiple phases of a nonlinear oscillator; frequency tripler by using a three stage oscillator; ring-type phase-based oscillators; voltage controlled oscillators and combinations thereof. Nowadays, microelectronics have developed to the point where radiation within terahertz frequency can be generated and used at least by propagating a lower frequency signal to transceivers which multiply the frequency up to the desired frequency range.
An example of this “propagation” approach is described by Ginsburg, et al., in US Pub. No.: 2012/0062286 entitled “Terahertz Phased Array System.” Phase array systems have become commonplace, having several uses, the most common being for radar systems (i.e., pulse radar and Doppler shift radars). According to Ginsburg, Phased array radar has replaced most of the previous generations of mechanical sweep radar systems because there is a lower likelihood of failure due to wear since mechanic components are replaced with electronics and because the sweep rates are much higher.
Referring now to the drawings, FIG. 1 shows a block diagram illustrating the basic functionality of a conventional phased array system 10. System 10 generally comprises a signal generator 12, phase shifters 14-1 to 14-K, a phased array 16 that includes radiators 16-1 to 16-K, and a direction controller 18. Following Ginsburg, in operation, the signal generator 12 provides a signal that is to be transmitted (i.e., pulse for a pulse radar). Based on the desired direction, the direction controller 18 provides control signals to the phase shifters 14-1 to 14-K, which varies the phase of the signal provided to each of the radiators 16-1 to 16-K within the phased array. Because the signals transmitted through radiators 16-1 through 16-K are generally out-of-phase with one another, constructive and destructive interference of the radiated signal forms a beam in a desired direction.
Ginsburg explains further that these conventional systems have been limited to conventional radio frequency (RF) frequency ranges. He cites the example that the frequency range for conventional radar is between 3 MHz (for HF-band radar) and 110 GHz (for W-band radar). The reason for the use of these relatively low frequency ranges being that there has, historically, been an unavailability of compact semiconductor sources of coherent radiation at the terahertz frequency range (which is generally between 0.1 THz and 10 THz). Generally, electronics and oscillators in the microwave range run out of power gain with increasing frequency, and typical broadband infrared blackbody sources begin losing available power within this region. Use of terahertz radiation, however, is highly desirable because of its unique properties. Namely, terahertz radiation has properties of lower frequency radiation (i.e., microwaves) which can be generated electrically and higher frequency radiation (i.e., visible light) which can be controlled using optics.
Ginsburg goes on to explain that today there exists two general types of terahertz sources: incoherent source and coherent sources. The incoherent sources are generally broadband incoherent thermal sources, which includes ultra-short femtosecond pulsed laser exciting photo conductive antennas, nonlinear electro-optical crystals, or non-linear transmission lines that suffers from very poor conversion efficiency (1 W laser pulse produces broadband energy in the nW-mW range). The coherent sources are generally narrowband continuous wave (CW) coherent sources which include diode multiplying microwave oscillators, gas lasers using carbon dioxide laser pumping methanol or cyanic acid, optical down conversion by difference mixing, and semiconductor quantum lasing. These coherent sources, though, generally consume a large amount of power, are not compact, require exotic materials, and/or are expensive.
Now referring to FIG. 2, a phased array system 20 is shown. The phase array system 20 generally comprises a Local Oscillator, LO 22, a phased array 25, a distribution network 27, receiver circuitry 33, and controller 23. The phased array 25 generally comprises several transceivers 24-1 to 24-N arranged in an array. The distribution network 27 generally comprises amplifiers 26 and 28-1 to 28-N. Additionally the receiver circuitry generally comprises a summing circuit 30, a mixer 32, amplifier 34, filter 36, switches 38-1 to 38-N, variable selector 39, and Analog-to-Digital converters ADCs 40-1 to 40-N.
Following Ginsburg, in operation, phased array system 20 (which is generally incorporated into an integrated circuit or IC) can generate a short range radar system that operates in the terahertz frequency range (which is generally between 0.1 THz and 10 THz). To accomplish this, local oscillator 22 generates a high frequency signal FL01 that is on the order of tens to hundreds of gigahertz (i.e., 40 GHz, 50 GHz, 67 GHz, and 100 GHz.) and a pulse signal TPUSLE 29. The distribution network 27 then provides signal FL01 21 to each of the transceivers 24-1 to 24-N such that the signals received by each of transceivers 24-1 to 24-N are substantially in-phase. A controller 23 provides a control signal to array 25, which phase-adjusts the transceivers 24-1 to 24-N with respect to one another to direct a beam of terahertz frequency radiation. The transceivers 24-1 to 24-N can then receive reflected radiation back from a target, which is provided to summing circuit 30. The output of summing circuit 30 is the converted to a digital signal by a mixer 32, amplifier 34, filter 36, switches 38-1 to 38-N, variable selector 39, and ADCs 40-1 to 40-N. Additionally, mixer 32 can receive a divided signal from LO 22 (i.e., FL01/2 or another synthesized signal) or can be removed (typically for 40 GHTz or less).
According to Ginsburg, this phased array system 20 generally has several different types of operational modes: pulsed, continuous, and stepped frequency. For a pulsed operational mode, a pulse of terahertz radiation is directed toward a target. The continuous operational mode uses a continuously generated beam, which is generally accomplished by effective “shutting off” the pulse signal TPULSE 29. Finally, stepped frequency allows the frequency of the terahertz beam to be changed, which can be accomplished by employing a bank of local oscillators (i.e., 22). For the pulsed operational mode, in particular, Ginsburg discusses in US Pub. No.: 2012/0062286 the range of the system 20 as governed by the following equation:
      R    =                  σ        ⁢                                            PG              2                        ⁢            λ            ⁢                                                  ⁢            n            ⁢                                                  ⁢                          E              ⁡                              (                n                )                                                                                        (                                  4                  ⁢                                                                          ⁢                  π                                )                            3                        ⁢                          KTBF              ⁡                              (                                  S                  N                                )                                                        4        ,for which the various terms are defined in the cited publication and incorporated by reference here.
In contribution to these endeavors, a THz signal generation system is disclosed herein which is based on traveling wave oscillators providing orders of magnitude higher oscillation frequencies resulting in a unique THz transceiver system that can generate, transmit and sense THz frequency signals as described further below in Detailed Description section.